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DrX Whiz Niraj
Errors in Analysis: Accuracy & Precision
Comprehensive exam notes on Types of Errors, Methods to minimize them, Accuracy vs Precision, and Significant figures.
In pharmaceutical analysis, an Error is defined as the difference between the measured (experimental) value and the true (actual) value. No analytical measurement is perfectly exact; some degree of error is always present.
1. Determinate (Systematic) Errors
These errors have a definite value, an assignable cause, and they affect the result in a specific direction (either always high or always low). They can be minimized or eliminated.
- Personal Errors: Color blindness, wrong reading of burette (parallax error), spilling sample.
- Instrumental & Reagent Errors: Uncalibrated glassware, impure chemicals, faulty balance.
- Operational Errors: Improper washing, incomplete drying of precipitate.
- Method Errors: Flaws in the chemical method itself (e.g., incomplete reaction).
2. Indeterminate (Random) Errors
These arise from uncertainties in a measurement that are unknown and uncontrollable. They follow a random distribution.
- They cannot be identified or completely eliminated.
- Causes: Slight temperature variations, vibrations in the building, minor voltage fluctuations.
- Solution: Perform the experiment multiple times and take the average (mean) to reduce random errors.
Since determinate errors can be identified, we can use the following techniques to minimize them:
- 1. Calibration of Instruments: All apparatus (burettes, pipettes, weighing balance) must be calibrated before use.
- 2. Blank Determination: A titration performed without the sample (only solvent + reagents). The blank reading is subtracted from the actual reading to eliminate impurity errors from reagents.
- 3. Control Determination: Performing the analysis on a standard substance of known purity along with the unknown sample to verify the method's accuracy.
- 4. Parallel Determination: Performing two or three simultaneous analyses of the same sample. If the results are close, the error is minimal.
- 5. Standard Addition Method: Adding a known amount of pure standard to the sample to check the recovery and eliminate matrix interference.
- 6. Amplification Method: Used for very small quantities. The sample undergoes a reaction that multiplies the component to be measured, reducing titration error.
| Feature | Accuracy | Precision |
|---|---|---|
| Definition | The closeness of the measured value to the True (Actual) value. | The closeness of multiple measured values to each other (reproducibility). |
| Relates to | Correctness. | Consistency. |
| Affected by | Systematic (Determinate) Errors. | Random (Indeterminate) Errors. |
Definition: The number of certain digits plus one uncertain (estimated) digit in a measurement. It indicates the exactness of a measurement.
Rules for Significant Figures:
- All non-zero digits are significant. (e.g., 456 has 3 sig figs).
- Zeros between non-zero digits are significant. (e.g., 2005 has 4 sig figs).
- Leading zeros (before non-zero digits) are NOT significant. (e.g., 0.0025 has 2 sig figs).
- Trailing zeros in a number with a decimal point ARE significant. (e.g., 4.500 has 4 sig figs).
Why Does This Matter in Pharmacy?
In drug formulation and quality control, a small determinate error (like a faulty weighing balance) can lead to a severely overdosed or underdosed tablet. If a batch of tablets has High Precision but Low Accuracy, it means all tablets have the exact same weight, but that weight is entirely WRONG (e.g., 400mg instead of 500mg). This can be fatal to patients.